GFP-protein lethality

Hi,
I have a vector encoding for a fusion protein of GFP and a guanine
nucleotide exchange factor. NIH 3T3 cells are used to get stable
transfectants with this vector. About 10 days after selection with G418
individual green fluorescent cells are present, but when you have a look
at the cells when colonies are starting to arise ( contain around 15-20
colonies), the cells lost their green fluorescence.
If somebody has an idea of what is happening in these cells, and wants to
share his/her information, I would be very pleased.
Bianca.
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